Wednesday, July 6, 2022

Cadmium intake dropping in NZ

Dietary intake of the toxic cadmium metal in New Zealand has been declining slowly over the past 10 years, even though it continues to accumulate in soils fertilised by superphosphate. The cadmium build-up is slow and the levels pose no immediate concerns, Ministry for Primary Industries (MPI) scientist Gerald Rys said. The levels of cadmium in phosphate fertilisers have been much lower in the past decade than historic levels. The latest diet survey, in 2009, showed intake levels were well within acceptable levels established by the World Health Organisation and there was no cause for concern. NZ food products met all cadmium-related export standards. Diet figures could vary from soil levels because there were many factors affecting the uptake of cadmium in plants, Rys said. “It is not simply a case of cadmium uptake increasing in direct proportion to the gradual soil accumulation.’’ “It is not simply a case of cadmium uptake increasing in direct proportion to the gradual soil accumulation.’’ Gerald Rys MPI scientist

New Zealanders also ate a mixed diet, with about half the food being imported.

Rys is principal science adviser in MPI’s Science Policy Group and co-ordinator of the Cadmium Working Group involving government, regional government, and the fertiliser and farm industries.  

The management strategy includes the fertiliser companies having voluntary limits on levels of the metal in superphosphate.

This sets a maximum 280 parts per million cadmium to soil (280mg/kg of soil) and NZ actual levels were consistently well below this, he said. The average for the five years to 2005 was 175 parts per million.

Cadmium Working Group monitoring indicates average accumulation is about two-thirds of previous levels, at an estimated 4.3 micrograms per kg of soil a year, down from 6.6.

Potential newcomer to the fertiliser supply industry Chatham Rock Phosphate has reported that its potential supply area in the seas east of New Zealand has tested average cadmium rates of just 2.2 parts per million.

Mining of this resource was still several years away and there were limitations, such as a lower phosphate level, Rys said.

Cadmium inputs have reduced over time as higher-cadmium phosphate rock from Nauru was replaced by other sources, Landcare Research scientist Jo Cavanagh said.

“Good progress has also been made by the management group, with structures and processes in place, and good buy-in from the industry, so we are ahead of where we were.’’

However, long-term risks remained. In a report for MPI Cavanagh said further research was needed to better understand the pathway of cadmium accumulation in animal agriculture systems, to establish the relationship between plant uptake for specific species and cultivars, and also into the risk of cadmium leaching to groundwater, especially in stony soils.

Talks are under way on how this can be funded.

The research wasn’t urgent but the sooner the better, because this would provide more options for the working group, she said.

Taranaki and Waikato are the areas of NZ where cadmium levels in soils are the highest, because of their long period of dairy farming and related superphosphate use. Waikato has also been a long-term region for potato growing. Bay of Plenty, another dairying region, has the third-highest levels.

In 2008, the average levels in Taranaki and Waikato were about 0.6mg/kg of soil, with about 0.5mg/kg in Bay of Plenty.

For dairying and meat products it is understood the levels could be significantly higher before posing food safety issues.

However, there could be implications for subdividing farmland into residential or rural-residential developments.

A 2005 Environment Waikato report put the threshold at 1mg/kg, without some form of investigation and-or remediation process. It suggested some pastoral and horticulture land was already at that level and that an average 1mg could be reached in another eight to 16 years. If so, that period would be starting about now.

On pastoral and horticulture land with lower-than-average cadmium levels, still comprising more than half the Waikato farm land area, the 2005 report expected the 1mg/kg level would be reached in 40-60 years. 

These figures had been based on conservative estimates, Rys said, assuming cadmium levels remained at the maximum 280 parts per million of soil, rather than the lower amounts actually being used.

Cadmium build-up in soils was recognised as an issue in NZ in the 1990s. The working group was set up in 2006 and its current strategy is for the period to 2017, but the long-term goal is to ensure cadmium does not become a concern over the next 100 years.

In the red-meat industry studies have shown higher cadmium levels in offal than in meat-muscle food, so to meet export rules MPI requires that all kidneys of animals older than 30 months at time of processing be discarded.

This was a conservative risk-management system and compliance was at a high level, Rys said.

Many other agriculture countries where phosphate fertilisers were used regularly were involved in similar management strategies as NZ, Rys said.

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